Seasonal variation of dissolved oxygen in Sanya Bay

Dissolved oxygen concentration is central in determining the water quality level for an aquatic ecosystem. In this article, we analyzed in situ dissolved oxygen and other related parameters from a dataset observed during 2004–2010 in Sanya Bay to find out its status in recent years and what controlled its seasonal variation. Different to worldwide coastal and estuarine ecosystems, dissolved oxygen concentrations in the Bay reached minimum values in autumn rather than summer during most of the seven years, while maximum values were seen in winter. Results showed that the seasonal trend of dissolved oxygen was mainly controlled by physical instead of biochemical processes. While many harmful factors normally act to lower dissolved oxygen concentration during summer, such as high surface water temperature, strong stratification, and large loads of nutrients from the river basin, upwelling driven by summer monsoons cooled down the bottom water. This increased dissolved oxygen saturation and hindered biochemical oxygen demand. On the contrary, the bottom water was warmed up abruptly during autumn, when upwelling was reduced due to the cessation of summer monsoon. This decreased bottom water dissolved oxygen saturation and enhanced some biochemical processes. Overall, dissolved oxygen values were greater than 6.0 mg l⁻¹ during most years, indicating a healthy water quality environment in the Bay. Nevertheless, low dissolved oxygen bottom water conditions (<6.0 mg l⁻¹) still existed in the whole Bay during some seasons—like the summer of 2009—which indicates there is some concern for the current stage of water quality in the Bay.